This invention relates in general to sewing machines and in particular to a new and useful material guiding device which comprises a pressure pad holder which is pivotally supported adjacent the sewing needle and includes a pressure pad which is movable in the holder and a holder which is selectively movable to shift the pressure pad.
Known from German Pat. No. 25 22 422 is a sewing machine for the automatic sewing of such seams, there being provided on one side of the needle a guide ruler and on the opposite side of the needle a pressure pad. The drive mechanism for the pressure pad is a pneumatic cylinder, driven via a solenoid valve by a device scanning the edge of the material in outwardly curved material areas deviating from a straight line in order to lower the pressure pad onto the material to produce a curved seam parallel to the edge at a lateral distance from the needle and to push the pressure pad against its bearing surface to generate a braking force. Due to the continued operation of the material feeder the material is turned about the pressure pad as axis of rotation in accordance with the shape of the edge. The pressure must be great enough so that the tightest curve can still be sewn accurately in a workpiece to produce a seam parallel to the edge. But the consequence thereof is that all curves having a greater radius cannot be controlled parallel to the edge, or only when materials of great natural stiffness are involved. The spacing of the guide ruler from the needle must correspond to the desired spacing of the seam from the material edge, which follows from the tightest curve of the material contour. By the same token, the spacing of the pressure pad impact point from the needle must be in harmony with the tightest curve of the material.
Since in all larger curves the pressure pad impact point is outside of the center of the curve radius, the pressure exerted by the pressure pad can be overcome by the reactive force acting upon the material when the latter's edge is pushed, during the turning motion, against the guide ruler only when naturally very stiff materials are involved so that the material slips under the pressure pad transverse to the feeding direction away from the guide ruler. This means that curves with larger radii of curvature can be controlled parallel to the edge only when naturally stiff materials are being sewn.
To improve the control it has already been suggested (German OS No. 27 16 914) to control the force acting upon the pressure pad as a function of the size of the radius of curvature so as to increase with decreasing radius. But even with this type of contour control it is not possible to obtain a contour-correct seam parallel to the edge when soft materials are involved because the natural stiffness of the material is insufficient to transform the force which acts as torque when the pressure pad is lowered and the material feeder continues operating and which pushes the edge of the material against the guide ruler during the turning motion partly into a force to move it transversely under the lowered pressure pad away from the guide ruler so as to react upon the material. Due to its little natural stiffness the edge area of the material pushed against the guide ruler during the turning motion escapes upwardly at the guide ruler and is placed in a vertical position, often even rolled up.
In the area of curves having a radius larger than the smallest radius of curvature this results, in a stretched position, in a seam spaced further away from the edge than in straight edge areas or at the tightest curve of the material. When binding, the rolled up edge areas are enveloped by the binding tape and sewn in. This can falsify the outer contour of the material considerably in part.
To avoid the disadvantages mentioned, especially when processing materials of little natural stiffness, it has been proposed in German OS No. 30 48 198 to drive the pressure pad intermittently as a function of the angular position of the sewing machine main shaft so that the pressure phase of the pressure pad essentially coincides with the feeding phase of the material feeder of the sewing machine.
Driving the pressure pad intermittently achieves that braking force pulses are caused to act upon the material in rapid succession while the material is turned in smaller angular amounts by the material feeder, whereas no pressure is exerted in the intervals between each two pressure phases. In the process, the material can escape away from the guide ruler transverse to the material feeding direction to the same extent that its lateral edge is pushed against the guide ruler, without having to overcome a pressure pad resistance. While in most cases, the correction of the material position achieved by this measure suffices to obtain a satisfactory result, both the oscillatory frequency and the contact pressure imparted to the pressure pad would have to be adapted to the respective thickness of the material to be processed when material of greatly varying thickness in running sequence is being processed, if deviations from the desired result are to be avoided. This, however, is not feasible in practice because sewing tests have to be conducted on some workpieces to arrive at a new setting, causing considerable time losses.